AC maintainence questions
 

I have a 1996 e150 XLT van. I've had it 30 years and have never touch the air conditioning system. The compressor and clutch work fine but it wasn't blowing as cold as it used to, so I want to add some 134a. Can someone tell me what's the maximum I can safely put in the system. I've heard between 30 and 40 but I would like a more specific number and also I'm getting mixed messages about whether to add any oil to the system. Since I've never done any tune-up or maintenance to it I was under the impression it would not hurt to add some oil but then I've been told unless it's broken not to put any in it could somebody please give me some information here thank you Teresa